This invention relates to welding technology. More particularly, this invention relates to friction stir welding of metal components. This invention also relates to articles made using friction stir welding.
The continuing effort to design and build more powerful and more efficient turbo-machinery such as, for example, gas and steam turbines, requires the use of materials having enhanced high temperature performance capabilities. However, the performance enhancements in such exemplary properties as strength and creep resistance obtained in many state-of-the-art turbine materials often come at the expense of other properties, and a trade-off is observed. For example, many high-temperature nickel-based alloys show markedly increased susceptibility to cracking when joined by conventional welding technology. The melting and solidification associated with conventional welding causes localized degradation in the properties of many of such materials. As a result, these alloys are not usable in machine designs that require significant use of conventionally welded structures. The cost-effective assembly of complex machine components by welding sub-components made by casting or forging metal alloys is recognized to be desirable in the industry, and so the inability to use certain high-performance materials in such welded assemblies is a significant limitation.
Solid-state welding processes have been developed as a way of addressing this issue. These processes join metal components without melting, thereby avoiding the effects associated with traditional welding techniques. Typical solid-state techniques, such as inertia welding and translational friction welding, have been used as alternatives to conventional welding in the turbine industry, but these techniques are restricted to forming joints of simple geometries of relatively limited sizes. However, one solid-state welding technique, known as “friction stir welding” (also referred to herein as “FSW”) is capable of producing large welds in a variety of geometric configurations not available to other such processes. In this process, a non-consumable, rotating cylindrical tool is plunged into a rigidly clamped workpiece, and then traversed along the joint to be welded. The tool is specially designed to provide a combination of frictional heat and thermo-mechanical working to the workpiece material as the tool traverses along the joint. A strong, solid-state bond is formed in the wake of the tool.
Friction stir welding was developed originally by The Welding Institute, Great Abington, UK, for joining aluminum alloys that were difficult to weld by conventional processes. Although successful in joining these relatively soft materials, expanding the application of FSW to harder, stronger materials, such as, for example, nickel and titanium alloys, has proven to be difficult due to the propensity of the tool to degrade during the welding process. Much of the degradation may be attributed to the high heat and stresses generated during FSW of these high-strength materials.
Therefore, there is a need to provide welding tools, apparatus, and methods that are capable of reliably joining components made of high-strength materials by FSW. There is a further need for the articles made using these apparatus and methods.